Limnology (2+1)

1.2.1 Inland waters

Unit 1- Limnology- An introduction

1.2.1 Inland waters
The inland waters which include both fresh water masses and estuarine waters of varying salt content are clearly distinguishable from the salt waters of the oceans. The inland water masses are discrete and being isolated within the specific land area, acquire the characteristic chemical composition of the land, by exchange between soil and water. The oceanic water on the other hand is open and mixing together by wind action and currents and therefore more homogeneous in chemical composition. However, the land water exchange is limited to coastal areas. The estuarine waters are mixtures of sea and freshwater, but with the higher content of salts in the sea water (150 – 200 times that of freshwater), are dominated by the sea water effects.
According to Hutchinson (1959), limnology is the large variety, individual and groups of inland water bodies, the diversity being caused by the diversity of their origin as well as by the diversity of their chemistry and biology.

Types of inland water
Frey (1960) has classified inland waters in three different ways viz, depending on whether the water is stationery or flowing, depending on whether the water mass is natural or artificial and permanent / temporary.
I a. Flowing waters (Lotic waters)
These include creeks, streams and rivers mentioned in that sequence because of their sequence of succession also in the same order, through the natural processes of lengthening and widening of running waters. In these, there is continuous current of water in one direction. The organisms inhabiting these waters have complexity of adaptation towards the increase in water current speed. It includes all forms of inland waters in which the entire body of water moves continuously in a definite direction. The sequence of genesis is brooks, rivulets, channels and rivers.
b. Standing waters (Lentic waters)
Here, water current is not a major ecological factor; unlike in the lotic series lakes, ponds and swamps form the lentic series. The sequence indicates the natural evolution of water masses as well lake may either be productive or non-productive, when they are referred to as eutrophic or oligotrophic respectively. It includes all forms of inland waters – lakes, ponds, swamps and their various integrades in which the water does not flow continuously in definite directions. Essentially, the water remains standing, though a certain amount of water movement may occur, such as wave action, internal currents or flow of water in the vicinity of inlets and outlets.
The sequence of genesis is as follows Lake – pond – swamp.
a. Natural bodies of water
Certain parts of the world are endowed with an abundance of natural waters serving human needs.
b. Artificial bodies of water
According to man’s needs water bodies are created artificially. It includes ponds, wells, tanks reservoirs etc.

i. Ponds
In India, even from ancient times, large ponds and wells exist serving for drinking water and also for irrigation purposes.
Types of ponds
Based on seasonal duration ponds can be classified into two types.
1. Temporary ponds
2. Seasonal ponds
Temporary ponds divided into three types,
1. Vernal ponds: Water exists only in spring season.
2. Vernal Autumnal pond: Water exists in those ponds during spring and autumn and they dry in summer.
3. Aestival ponds: Water persists in these ponds throughout the season but it freezes during winter.
Permanent ponds
Water persist in these ponds throughout the season but it freezer in winter.
i. Reservoirs
Rivers are blocked and reservoirs or artificial lakes are developed. These serve in generating hydroelectric power, irrigation, fish production and recreation. These also help in flood control.
ii. Tanks
In India there are both perennial and temporary tanks. There are some in which water remains for 6-9 months duration, called long seasonal tanks and in some, water remains for less than 6 months, referred to as short seasonal tanks.

III a. Permanent waters
In most parts of the world where there is precipitation exceeds, the evaporation and seepage loss the waters in rivers, lakes and ponds and are termed as permanent waters.
b. Temporary waters
Evaporation loss of water is more than the precipitation gains, as in all arid areas of the world, the water bodies dry up usually during summer. In high latitude (30-50o), when the area is desert the rivers and streams drain into permanent or temporary lakes. These lakes have salt differing from that of sea, the salt washed down to the lakes being predominantly potassium and sodium carbonates and magnesium and sodium sulphates and not sodium chloride. By evaporation at times salt concentrations in these waters exceeds that of the sea. (eg. Great salt lakes and Dead sea). The salt concentration of Dead sea is so high that there is no life in it.